Many electrical devices require high-performance contacts in which contact resistance should be low. Such contacts are used extensively in plugs, pins, relays, integrated circuit connectors, and the like. A typical specification for contacts used in connectors for electronic equipment includes a requirement for a contact resistance of less than 50 milliohms (m.OMEGA.). In addition, the contact should be resistant to atmospheric corrosion, and should be able to maintain its properties through a large number of operating cycles.
One common type of connector used on removable integrated circuit boards and the like is the "wiping connector", in which two contact surfaces "wipe" against each other as the connection is made. Such wiping contacts are generally located on the edges of the boards, and at least partially clean themselves when the board is inserted into a corresponding receptacle. Another type of connector is the "zero insertion force" (ZIF) connector, in which a first contact surface moves normal to a second surface to make contact without any wiping action. This type of connector can be located anywhere on the surface of an integrated circuit board, and thus offers greater flexibility in circuit design.
Precious metals, such as gold, platinum, and palladium have been found particularly suitable as contact materials because of their low contact resistance, chemical inertness, and reasonable abrasion resistance, particularly when alloyed with hardening additives. Contacts using precious metals often consist of a conductive substrate of a less expensive metal, such as copper or nickel-coated copper, on which the precious metal is applied to provide the contact surface. For example, one type of widely used gold electrode comprises a copper substrate, with a nickel intermediate layer, and a 25 microinch (0.6 .mu.m) cobalt-hardened gold finish.
Becaust of the high cost of precious metals, the amount of such metals used in contact is an important consideration. Typically, a gold surface layer is at least 0.6 micrometers thick to ensure low porosity, low electrical resistance, and high wear resistance. Significant cost savings could be achieved by using a relatively inexpensive non-precious metal in place of some or all of the precious metal in contacts. However, non-precious metals have been found to be less reliable than the precious metals for precision contact surfaces. For example, although nickel has been used as a contact surface material in some types of devices, its susceptibility to oxidation, and the resulting increase in electrical resistance, has prevented its use on high performance contacts. (See "Properties of Electroplated Nickel Alloy Films for Contacts", by M. Robbins et al, Plating and Surface Finishing, March 1987, pages 56-59; "Stability of Electroplated Ni Films as a Function of the Electrolyte", by M. Robbins et al, Extended Abstracts of the Electrochemical Society, Fall Meeting 1987; and Nickel and Chromium Plating, by J. K. Dennis and T. E. Such, Butterworths, London, second ed., 1986, for detailed descriptions and characterizations of electrodeposited nickel films.) (See also U.S. Pat. No. 4,518,469, issued May 21, 1985 to Ng et al., for a method for electroplating an alloy of nickel and antimony from an acidic solution onto a contact substrate.)
In usual practice, steps are taken to ensure that the surface of a contact has a bright, shiny finish rather than a dull or matte finish. A bright finish is cosmetically more acceptable, and is also preferred because a dull finish generally indicates oxidation, porosity or other impurities or disruptions in the surface. However, Gamblin, U.S. Pat. No. 4,564,565, issued Jan. 14, 1986, relates to a method of making matte-finish electrical contact surfaces by the electrolytic deposition of nickel in crystalline form onto a substrate. The process involves deposition from a plating bath containing a nickel salt and a specific anion selected from the group of TiF.sub.6, ZrF.sub.6, HfF.sub.6, and TaF.sub.7.
The presence of contaminants on the surface of contacts is associated with greatly increased contact resistance of the connector, regardless of the conductivity of the underlying material. Although chemical inertness generally prevents the formation of oxides or other decomposition products on precious metal-coated surfaces, oxidation of non-precious metals, as previously discussed in the context of nickel, has been a problem. Such oxidation typically forms a strongly adherent insulating layer which increases contact resistance. In addition, the accumulation of loose airborne contaminants, such as hydrocarbons, salts, fine dusts, and the like, tends to increase the contact resistance of any contact. Although the wiping action of wiping contacts can generally remove loose surface contaminants, tightly adhered oxidation layers are not so easily removed. Furthermore, ZIF-type connectors need to be able to form low contact resistance. connections without any wiping action.